Wireless communication links are known to be sensitive to time-varying propagation conditions. Multiple propagation paths with different attenuation and delay characteristics produce an effect known as fading. Fading in millimeter wave communication links may also be caused by rain attenuation. In some communication links, co-channel and adjacent-channel signals introduce interference which degrades the quality of a received signal. These phenomena, which are time-varying, reduce the Signal to Noise Ratio (SNR) obtained at a receiver output.
A mathematical measure of the data rate which may be transmitted over a given channel with a given, arbitrarily low, error probability is the channel capacity. The channel capacity is proportional to a logarithm of channel SNR. An SNR increment of 3 dB enables increasing a data rate by approximately 1 bit per Hz. Some modern communication systems try to maximize their data rate by transmitting at a data rate which tracks the channel capacity. For such a purpose a communication channel SNR estimator is typically included. The modulation type and coding rate of the communication system are typically selected from a predetermined set according to an estimated SNR and possibly to adaptive SNR threshold levels. Such systems will be referred to herein as Adaptive Coding and Modulation (ACM) systems.
Typical operation of an ACM system is as follows. A receiver estimates the channel SNR or some other indication of communication quality. If the estimate of the quality is sufficiently high such that upgrading of the communication rate is possible, a request for an appropriate coding/modulation scheme is sent to the transmitter on a return link. A protocol for scheduling the modulation/coding switch should attempt to provide an errorless transition to the new modulation. A similar procedure takes place when communication quality gets dangerously close to a threshold of operation in which intolerable errors may occur. In such a case, transmission switches to a lower communication rate.
For fast fading channels it is desirable that the SNR be measured with high accuracy and with a low delay. Typically the measurement is done in a decision directed manner. A Mean Squared Error (MSE) of the signal at the demodulator output is calculated via hard decisions regarding the transmitted symbols. The MSE may then be transformed into an estimate of the SNR.
Occasionally, despite efforts made by a system to track the channel capacity, a selected modulation scheme is too large, that is, has too many bits per symbol for the channel to handle. Whichever is a weakest point of the receiver is a first point to break down. In millimeter-wave communication systems, the weak point is typically a carrier phase tracking mechanism. As a result, demodulator output also breaks down, and an estimated MSE indicates an SNR significantly worse than actual SNR. Such a system typically responds by switching to a very low bit rate, sometimes lower than necessary, thereby wasting bandwidth.
The disclosures of all references mentioned above and throughout the present specification, as well as the disclosures of all references mentioned in those references, are hereby incorporated herein by reference.